Several reasons.
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The philosophical. This is something that is unique and took billions of years to evolve to its present state. There’s nothing else like it in the universe.
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The practical. It plays a unique part in the ecological web of life. In many cases organisms are reliant on other organism for dispersal, reproduction etc. The removal of a species is like pulling a Jenga block from the ecosystem.
Earth’s eco system is awesome. The chances of any species existing in its current for is minuscule. We should mourn their passing.